Geoid Models

A geoid model is a reference model for physical heights – defined as a surface for which the physical (orthometric-) height is zero. For a given location, the model is commonly expressed as height (in meters) above the reference ellipsoid along the ellipsoidal normal for this location. The geoid model is a synthesis of various geodetic sources of information and measuring techniques: GNSS positioning, spirit levelling, Digital Elevation Model, topographic density, global Earth Geopotential Models and the local surface gravity information. The consistency of the geoid model across all these different sources of information is a measure of its quality.

In Denmark, the standard deviation of the difference between the gravimetric geoid model and the heights obtained from spirit levelling is less than 0.02m. For the “fitted geoid” (a sum of gravimetric geoid heights and a modelled correction surface) this number is less than 0.01 m. The corresponding numbers for other countries in the vicinity (which often are much bigger, have more topographic relief and are more affected by the post-glacial land uplift) are less than 0.04 cm and better for the “fitted geoid”. Maintenance and the quality assessment of the input information for geoid modelling is a continuous and ongoing activity. Temporary geoid models are regularly computed often in connection with broad regional efforts.  It is the national mapping authority that upgrades a specific (“fitted”) geoid model to the official height reference model for the country.

In Greenland the situation is much more challenging. The size of Greenland is comparable to that of a continent with vast, remote and inaccessible areas. The center of Greenland is an ice cap which melts and where the ice calves at the coast creating locally a land uplift. There is no national spirit levelling. Only local spirit levelling in main municipalities exists. Nevertheless, Greenland is important for studies of climate change globally and a global laboratory for various nouvelle remote measurement techniques (satellite, airborne, helicopter, ... etc.). The surface gravity data for the ice-free coastal zone exist (mostly helicopter surveys), as well as few gravity transects across the ice cap, but most of the surface of Greenland is covered by gravity data obtained by remote sensing techniques. One other modelling challenge for geoid determination is the fjords. The overall quality of the geoid of Greenland is probably at the decimeter level and much better in better surveyed municipality areas.   

The above experience from Greenland led to numerous international geoid modelling projects across the globe, where the task was to use the local geodetic information and possibly to enhance it by airborne surveys (often to cover remote and inaccessible parts of the country) and to obtain as good geoid model as possible. This effort is often accompanied by training the local stuff of the national mapping agencies to maintain the geoid for the future.